Devices for calculating and displaying the loads supported by cranes, derricks, and the like, have long been used as operator aids in preventing unstable conditions or the overstressing of structural elements in the crane boom. This capability is particularly important in mobile cranes of the type having telescopingly extendable booms which can be slewed through the whole or part of a circle during normal operation. By comparing the load indication of an operating aid with the load rating table supplied by the crane manufacturer for a specific crane and operating configuration, an operator can determine the relative stability of the crane. Typically, two methods of determining the load supported by the crane have been employed.
The first method involves the direct measurement of the actual weight of the load by devices such as tensiometers, strain gauges, and the like.
The second method involves the calculation of the total effective hook load, which is determined by first calculating the total turning moment of the boom and load about the boom pivot pin. By dividing the total turning moment by the horizontal radius of the load from the pivot pin, the total effective load can be calculated.
With both methods, the actual load or total effective load can thus be determined and displayed to the operator who, upon referral to the load rating tables, can determine the amount of crane lifting capacity remaining at any given time.
In order to calculate the total turning moment of the boom, it is necessary to determine the reaction forces generated by the boom and load upon the structural elements of the crane which are supportive of the boom. In cranes with luffable booms, these supporting elements are typically cables or hydraulic lift rams. In the case of cables, tensiometers are frequently employed to measure axial forces exerted by the boom and load. In cranes having lift rams, hydraulic pressure within the ram has been used as a measure of reaction force along the axis of the ram.
A major shortcoming in prior art arrangements occurs in cranes employing multiple lift rams which act upon the same reaction force. All such rams tend to leak hydraulic fluid over time when under load. Multiple rams typically leak at differing rates, a phenomenon which results in different pressures being present in the rams. This variance of pressures between rams typically was not accounted for in prior schemes and can result in extremely inaccurate reaction force measurement. When a crane remains stationary in a loaded condition for an extended period of time such as overnight or is subjected to significant side loading during operation, the pressure difference between its rams can become so large as to produce a grossly inaccurate or dangerously misleading (understated) load indication.